Spatial regulation of AtABCG25, an ABA Exporter, Is an Important Component of the Mechanism Controlling Cellular ABA Levels

Abstract

The phytohormone abscisic acid (ABA) has important roles in various physiological processes, such as dormancy, development and response to biotic and abiotic stresses, in plants. Recently, multiple ABA transporters including importers and exporters were identified helping to understand ABA transport system within the cells and long distance transport of ABA. In this study, we examined the internalization and recycling of an ABA exporter, ATP-BINDING CASETTE G25 (ABCG25), by genetic and biochemical approaches. ABA levels in the cells can be regulated by transport system as well as biosynthesis pathways and catalytic pathways. However, it is not known how ABA transporters are controlled under diverse conditions to maintain ABA homeostasis. Here, we report that spatial regulation of ABCG25 is an essential mechanism controlling its activity. ABCG25 localized to the plasma membrane and was subject to post-translational regulation via clathrin- and adaptor protein complex-2 dependent endocytosis followed by trafficking to the vacuole for its degradation. The levels of ABCG25 at the plasma membrane were regulated by abiotic stresses and exogenously treated ABA; the endocytosis of ABCG25 was activated under abiotic stress in an ABA-independent manner while the recycling of ABCG25 was enhanced from early endosomes to plasma membrane upon application of exogenous ABA. Based on these results, we propose that the spatial regulation of ABCG25 is an important component of the mechanism by which plants fine-tune cellular ABA levels according to cellular and environmental conditions.